Molecular dynamics in binary organic glass formers

We investigated binary low-molecular-weight glass formers as model systems for mixtures of small and large molecules. Tricresyl phosphate (TCP) in oligomeric styrenes (OS), benzene in OS and polystyrene (PS), and benzene in TCP were studied by applying dielectric spectroscopy as well as H-1, H-2, and P-31 NMR spectroscopy. Temperatures above and below the glass-transition temperature (TG) are covered. The dielectric loss of the small component appears broader the higher the molecular ratio Mim is, and the lower the TCP concentration and the lower the temperature are chosen. Close to T-G, extremely broad distributions of correlation limes G(log tau) result, which are similar to those reported in the cases of polymer-plasticizer systems, although for our systems the motional heterogeneities are already established at similar M and m. By applying H-1 and H-2 (1D and 2D) NMR on benzene in OS and PS, we can demonstrate that the large molecules basically behave as in neat glass formers. However, the small molecules exhibit an isotropic reorientation also well below T-G, and the dynamics is rather characterized by a random jump process than by rotational diffusion, the latter being found in neat systems. Furthermore, we can prove that within G(log tau), exchange processes lake place, even below T-G, which essentially occur on the same time scale as reorientation.